The present invention relates to a temperature sensor for determining static temperature of air through which an air vehicle is moving by heating the sensor to establish a thermal offset for a temperature sensing element carried in a probe and designed to maintain substantially the same offset as airspeed changes within useful operating limits.
Measurement of the total temperature of air in which air vehicles are operating is well understood, and it is known that the total temperature increases as Mach number increases. Several types of total temperature sensors are used for measuring this parameter. Most sensors use a strut mounted probe that extends into the air stream so air flows through ducts in the probe, in one of which a temperature sensor element is mounted. Generally, efforts are made to ensure that the boundary layer of air on surfaces of inlet air scoops or ducts of the probe does not interfere with the sensed temperature. Heaters are often used on the air scoop ducts and probe for deicing purposes. The de-icing heaters are positioned so that the heating effect, or influence on the temperature measured, is minimized. The probe or strut housings may also have shields to minimize stray heat, but there always is some de-icing heater error. The deicing heater error increases with decreasing airflow in current total temperature sensors.
The relationship between total temperature and static temperature is known to be a function of Mach number. Static temperature is typically the temperature of “still” or undisturbed air, and total temperature is the temperature which would be realized if relatively moving air could be adiabatically brought to rest, for example, temperature of the air layer at rest along the stagnation line of an airplane wing. In other words, all of the kinetic energy imparted to the air results in the air being warmed to the “total temperature”. The higher the Mach number, the higher the total temperature for a given value of static temperature.
In practice, directly measuring true total temperature is never achieved because there is always some heat loss. A total temperature sensor acts much like the stagnation line of the wing but is optimized to minimize heat loss. A good total temperature sensor comes very close to indicating the true total air temperature.
On the other hand, there is no accurate way to measure static temperature directly on aircraft. Instead, it is easier to measure total temperature and use a separate pressure sensing probe to measure the total pressure and the static pressure, and then calculate static temperature from these measurements. This may, in fact, be the preferred approach if measurements of multiple air data parameters are required. If only a static temperature measurement is needed, however, pressure measurement instruments to provide static and total pressure inputs for determining static temperature add unnecessary weight, drag and complexity to the measurement system.
The present invention teaches obtaining static temperature (Ts) from a probe measuring the total temperature (Tt) without requiring separate pressure measurements.